1) Field of the Invention
The present invention relates to an electronic control unit for a vehicle, and more particularly to a vehicle passenger detection apparatus designed to make a decision on a state of a passenger or occupant (including a driver) sitting on a seat in a vehicle for transmitting the passenger state to a vehicle passenger protection device.
2) Description of the Related Art
So far, there has been proposed a passenger detection apparatus for a vehicle in which, for the purpose of changing a spread quantity of an air bag according to type of passenger, distortion type load sensors are located at a plurality of portions on a seat rail of the vehicle to measure a load (that is, a weight of the passenger) on a vehicle seat for making a decision on, in addition to the presence or absence of a seated passenger, the type of the seated passenger such as adult/child.
For example, as shown in FIG. 7, a conventional vehicle passenger detection apparatus 101 is principally made up of a power supply circuit 11, a signal voltage comparator 12, a microcomputer 130, a timer circuit 18, a current dissipation cutting circuit 14, a circuit 17 requiring a large current dissipation (consumption), and others. The power supply circuit 11 is connected through a power supply line 21 to a vehicle battery 2 to output a 5-V voltage through its VO terminal. Moreover, the vehicle battery 2 is connected through an ignition key switch 3 to the signal voltage comparator 12.
When the vehicle battery 2 is connected to the vehicle passenger detection apparatus 101, the power supply circuit 11 is powered through the power supply line 21. The power supply circuit 11 regulates the battery voltage to supply a 5-V voltage to the microcomputer 130 and the timer circuit 18. In response to the supply of the 5-V voltage, the microcomputer 130 starts its operation and works in accordance with a clock signal from a crystal oscillator 15. The microcomputer 130 implements predetermined arithmetic operation and control and then stops the generation of the clock signal in the crystal oscillator 15 for reducing the current dissipation.
On the other hand, the timer circuit 18 starts its counting operation in accordance with a clock signal from a CR oscillation circuit 16. When the count of a preset timer activation time reaches completion, the timer circuit 18 outputs a timer signal representative of the satisfaction of a timer activation time count condition to generate an external interrupt to the microcomputer 130. Upon receipt of the external interrupt, the microcomputer 130 starts the clock operation through the use of the crystal oscillator 15 which has so far been placed into a stopped state. Moreover, when needed, it turns on the current dissipation cutting circuit 14 to supply the voltage through an external power supply terminal 23 further to the external circuits including load sensors 31 to 34. On the other hand, the microcomputer 130 receives load detection values from the load sensors 31 to 34 through an external signal input terminal 24 and, when making a decision on the basis of the load detection values therefrom that a vehicle seat is in an unoccupied state, implements the zero-point correction (correction of the load detection value in an unoccupied state) on the load sensors. After the completion of the zero-point correction on the load sensors, the microcomputer 130 turns off the current dissipation cutting circuit 14 and stops the clock generation of the crystal oscillator 15 for the reduction of the current dissipation, and conducts the counting operation through the use of only the timer circuit 18.
In response to the turning-on of the ignition key switch (IG-SW) 3, a battery voltage is applied through an IG terminal 22 to the signal voltage comparator 12. When the inputted voltage to the signal voltage comparator 12 exceeds a reference (Ref) voltage, the signal voltage comparator 12 outputs an IG-SW signal to make an external interrupt on the microcomputer 130. Upon receipt of the external interrupt, the microcomputer 130 starts the clock operation through the use of the crystal oscillator 15 which has been in a stopped state. Moreover, it turns on the current dissipation cutting circuit 14 to supply the voltage through the external power supply terminal 23 further to the external circuits such as the load sensor 31. The microcomputer 130 receives the load detection values from the load sensors 41 to 34 through the external signal input terminal 24 and, on the basis of the load detection values, makes a decision, for example, on the presence or absence of a seated passenger and on whether the passenger is an adult or .child. This decision result is outputted through an external signal output terminal 25 to an air bag ECU 40. Incidentally, the timer circuit 18 continues the timer counting operation (timer count) even during the IG-ON (turning-on of the ignition key switch 3).
When the ignition key switch (IG-SW) 3 is turned off, the voltage at the IG terminal 22 becomes 0V. Thus, the inputted voltage to the signal voltage comparator 12 becomes below the Ref voltage and, hence, the signal voltage comparator 12 stops the output of the IG-SW ON signal. In response to the output stop of the IG-SW ON signal, the microcomputer 130 terminates the passenger state decision processing and turns off the current dissipation cutting circuit 14 and stops the clock operation through the crystal oscillator 15.
Accordingly, in the conventional vehicle passenger detection apparatus 101, during the stand-by, the timer circuit 18 produces a clock through the CR oscillation to carry out the timer counting operation at a low frequency for achieving the low current dissipation, while in the timer activated condition, the microcomputer 130 conducts the clock operation through the crystal oscillator 15 so as to surely make the zero-point correction on the load sensors 31 to 34.
There is a problem which arises with the above-described conventional vehicle passenger detection apparatus 101, however, in that, since timer circuit 18 produces a clock through the CR oscillation to conduct the timer counting operation and supplies a timer signal to the microcomputer 130 at the satisfaction of a timer activation condition to activate it, the timer count error increases in a case in which the CR oscillation frequency varies due to factors such as temperature variations or deterioration. In addition, there is a case in which due to the timer count error difficulty is encountered in carrying out the zero-point correction on the load sensors in a constant cycle, which can make it difficult to implement the passenger decision with high accuracy.